Two-probe theory of scanning tunneling microscopy of single molecules: Zn(II)-etioporphyrin on alumina

We explore theoretically the scanning tunneling microscopy of single molecules on substrates using a framework of two local probes. This framework is appropriate for studying electron flow in tip/molecule/substrate systems where a thin insulating layer between the molecule and a conducting substrate transmits electrons non-uniformly and thus confines electron transmission between the molecule and substrate laterally to a nanoscale region significantly smaller in size than the molecule. The tip-molecule coupling and molecule-substrate coupling are treated on the same footing, as local probes to the molecule, with electron flow modelled using the Lippmann-Schwinger Green function scattering technique. STM images are simulated for various positions of the stationary (substrate) probe below a Zn(II)-etioporphyrin I molecule. We find that these images have a strong dependence on the substrate probe position, indicating that electron flow can depend strongly on both tip position and the location of the dominant molecule-substrate coupling. Differences in the STM images are explained in terms of the molecular orbitals that mediate electron flow in each case. Recent experimental results, showing STM topographs of Zn(II)-etioporphyrin I on alumina/NiAl(110) to be strongly dependent on which individual molecule on the substrate is being probed, are explained using this model. A further experimental test of the model is also proposed.Comment: Physical Review B, in pres

Structure of the specific combining ability between two species of Eucalyptus. II. A clustering approach and a multiplicative model

International audienceThe Eucalyptus breeding program of URPPI (a partnership between CIRAD-Fore t, Centre National de la Recherche Forestie re du Congo, and Unite dÕAforestation Industrielle du Congo) consists of a re- ciprocal recurrent selection scheme developed in the Congo between the two species Eucalyptus urophylla and Eucalyptus grandis. Two approaches are proposed in order to model and predict the speciÞc combining ability (SCA) between these species. The clustering ap- proach uses a simultaneous clustering procedure of the two species based on SCA and reveals heterotic groups coherent with the geographical origins of E. urophylla genotypes. The second approach uses a multiplicative model to partition the SCA into three multiplicative terms explaining 95% of the interactio

Computation of Scanning Tunneling Microscope Images of Nanometer-Sized Objects Physisorbed on Metal Surfaces

This communication deals with the application of a transfer-matrix strategy for the quantitative evaluation of the tunnel current in a scanning tunneling microscope (STM). The image given by a simple atomic-size object deposited on a metal surface is specifically examined in both modes of STM operation namely the constant-height and the constant-current modes. The two-dimensional corrugation induced at low temperature by Xe atoms physisorbed on an otherwise clean, unreconstructed Ni (110) surface is studied in detail. It is shown that the simple consideration of the elastic scattering of electrons by the three-dimensional potential barrier between the tip and the metal substrates provides a quantitative description of the images produced by the instrument: (1) the Xe atom appears as a conic protrusion, approximately 7 A wide, with a corrugation 1.3 A high; (2) in Xe clusters, each adjoining atom is resolved, with a shape in full agreement with experiment. In order to obtain correct quantitative results, image-charge corrections to the potential cannot be neglected

Structure of the specific combining ability between two species of Eucalyptus. I. RAPD data

International audienceWithin the context of the reciprocal recurrent selection scheme developed in 1989 by CIRAD-Fore t on Eucalyptus, RAPD essays were performed to assess the genetic diversity in the two species E. urophylla and E. grandis. The molecular markers were split into two parts: the speciÞc markers (present with di¤erent fre- quencies in the two species) and the common markers (present with similar frequencies in the two species). The study analyses the structure of genetic diversity within and between the two species of Eucalyptus. Dif- ferent genetic distances are worked out for use in pre- diction equations of the individual tree trunk volume of hybrids at 38 months. Each distance is expressed as the sum of the general genetic distance and the speciÞc genetic distance. The general genetic distance based on the double presence plus the double absence of bands seems to be an interesting co-variate to use in a factor regression model. Through this model the distance calculated between species explains the general com- bining ability (GCA) and the speciÞc combining ability (SCA) of the phenotypic character with a global coe¦c- ient of determination of 81.6

Effet du gène de nanisme lié au sexe dw sur la croissance du poulet: étude biochimique et anatomique

L’allométrie de croissance postnatale de l’eau et des lipides corporels totaux a été étudiée chez des poulets porteurs [dw] ou non porteurs [Dw] du gène de nanisme. Chez les mâles [Dw] nous observons une accumulation isométrique de l’eau ou des lipides, alors que chez les [dw], l’allométrie de croissance est légèrement majorante pour les lipides, minorante pour l’eau. Il en est de même chez les femelles jusqu’au poids vif de 1 400 g chez les [Dw] et de 700 g chez les [dw]. Les lipides évoluent par la suite selon une allométrie majorante, l’eau selon une allométrie minorante, avec des changements de pente moins marqués chez les [dw]. Une étude de l’allométrie de croissance des tissus et organes a été menée chez les poules. Chez les [dw], le tibia et le tarsométatarse ont une croissance relative inférieure, et le tissu adipeux dissécable se développe sans changement d’allométrie marqué. Pour les autres tissus ou organes, les pentes des droites d’allométrie ne sont pas significativement différentes entre poules [Dw] et [dw].Postnatal growth allometry of body total water or lipids was studied for brother or sister chicks of dwarf [dw] or normal [Dw] genotypes. For [Dw] males, isometric accretion of water or lipids occurs throughout growth whereas it is slightly increasing for lipids and slightly decreasing for water in [dw] males. Similar allometric coefficients are founded in hens till live body weight of 1 400 g in [Dw] and 700 g in [dw]. Thereafter lipids accumulate more actively and water less actively, the slope changes of allometric straightlines being less pronounced in dw hens. Growth allometry of tissues and organs was also determined in hens. It is noticed that in [dw] hens, tibia and tarsometatarse present a lower relative growth and that dissectable adipose depots are developing without allometric changes. For other organs and tissues, allometric coefficients of straightlines are not significantly different between [dw] and [Dw]

Affine convex body semigroups

In this paper we present a new kind of semigroups called convex body semigroups which are generated by convex bodies of R^k. They generalize to arbitrary dimension the concept of proportionally modular numerical semigroup of [7]. Several properties of these semigroups are proven. Affine convex body semigroups obtained from circles and polygons of R^2 are characterized. The algorithms for computing minimal system of generators of these semigroups are given. We provide the implementation of some of them

Optical structure and function of the white filamentary hair covering the edelweiss bracts

The optical properties of the inflorescence of the high-altitude ''Leontopodium nivale'' subsp. ''alpinum'' (edelweiss) is investigated, in relation with its submicrometer structure, as determined by scanning electron microscopy. The filaments forming the hair layer have been found to exhibit an internal structure which may be one of the few examples of a photonic structure found in a plant. Measurements of light transmission through a self-supported layer of hair pads taken from the bracts supports the idea that the wooly layer covering the plant absorbs near-ultraviolet radiation before it reaches the cellular tissue. Calculations based on a photonic-crystal model provides insight on the way radiation can be absorbed by the filamentary threads.Comment: 9 pages, 13 figures. Published pape